A screen transmission system, which transmits image information of a computer's screen from a server to a client via a network, is known. In order to reduce an amount of data of communication between the server and the client, an art that the server transmits image information included in a differential region corresponding to a region which displays contents of a present screen changing from contents of a previous screen, and an art of coding the image information included in the differential region are applied to the screen transmission system.
PTL 1 describes an art for determining priority of each differential region and controlling an order of transmitting differential regions according to the priority. As an example of a method for determining the priority, PTL 1 describes that the priority is determined on the basis of pixel values of pixels included in the differential region and an area of the differential region.
Moreover, it is conceivable that, for example, a region which displays a character, an icon or the like out of in-image regions has semantic importance. As a usual method for specifying the character displaying region, for example, a method for specifying the character displaying region on the basis of smallness in number of used colors, a histogram, edge density or the like is conceivable. Moreover, it is also conceivable to specify the character displaying region on the basis of variance or contrast of signal values (pixel values) of the region.
As a related art for specifying the character displaying region, an art to detect the character displaying region by use of a telop (television opaque projector) (for example, refer to PTLs 2, 3 and 4). According to an art described in PTL 2, a user designates a character region on an image to generate a region designating signal which indicates the character region. According to an art described in PTL 3, information, which indicates similarity to the telop, is generated on the basis of a differential value between the maximum luminance and the minimum luminance. According to an art described in PTL 4, a region including the telop is determined according to density of edge.
As a usual method for a binarization process, there is a method for carrying out the binarization by calculating a threshold value on the basis of a histogram.